Oil-impregnated sintered bearings are a kind of plain bearings, obtained by impregnating a lubricating oil into porous bodies prepared by press molding metallic powders, typically copper powders, iron powders, tin powders, zinc powders, etc. followed by heating and sintering, and used in a self-lubricating mode. The oil-impregnated sintered bearings are cheap, but have a comparatively low friction, a high precision, and are used in a self-lubricating mode, and thus are widely utilized as motor bearings in any desired positions of driving parts for automobile electrical parts, audio-visual appliances, business machines, home electrical appliances, and computer auxiliary memory devices.
Characteristics required for lubricating oil for the oil-impregnated sintered bearings are, for example, good compatibility with bearing materials without any generation of corrosion, sludges, etc.; good applicability in a wide temperature range such as less evaporation loss and good stability against oxidation at high temperatures, less deterioration of fluidity at low temperatures, etc.; good rust prevention; and substantially no adverse effect on resins, a low friction coefficient, a good wear resistance, etc. to meet the recent trends towards down-sizing, lower electric current, and longer durability. Furthermore, the lubricating oil for the oil-impregnated sintered bearings must be able to maintain its performance stability for a long time, because the lubricating oil must be used up without any additional oil supply until the life of service parts comes to an end.
It is particularly important for the lubricating oil for use in oil-impregnated sintered bearings from the viewpoint of the compatibility with bearing materials that the lubricating oil to be impregnated into the porous bodies is forced to contact and coexist with various kinds of metal particles, etc. having a very large surface area, because the oil-impregnated sintered bearings are based on porous bodies prepared by press molding several kinds of metallic powders, and thus can be used stably for a long time without any metal corrosion and sludge generation.
So far proposed lubricating oils or lubricants for oil-impregnated sintered bearings comprise an ester-based oil having a distinguished high-temperature lubricability as the main component of base oil, and a phosphorus-based extreme pressure agent or a sulfur-based extreme pressure agent to suppress frictional abrasion or shaft loss, thereby satisfying such requirements as down-sizing, higher speed, lower electric current, and lower power consumption.                Patent Literature 1: JP No. 3,433,402        Patent Literature 2: JP-A-2001-323293        Patent Literature 3: JP-A-2002-180078        
Among the extreme pressure agents, however, there are not a few agents that corrode bearing metals or generate sludges, etc. The resulting corrosion, sludges, etc. will clog up pores of bearings, so the lubricating oil cannot be fully fed to the sliding parts to cause cutting of oil films and lubricating failure, resulting such fatal cases as motor stop within a considerably shorter time than expected. Such inconvenience leads to no more attainment of the characteristics proper to the lubricating oil for oil-impregnated sintered bearings such as self-lubrication and maintenance-free operation.
To ensure practical use in a wide temperature range, the conventional mineral oils, and various synthetic oils such as synthetic hydrocarbon oil, ester-based oil, and fluorocarbon oil have been so far used. Furthermore, to ensure the low evaporation or stability against oxidation at high temperatures, and to maintain the oil film even at high temperatures, a viscosity index improving agent has so far been used. The ester-based oil has a distinguished high-temperature lubricability, but has an adverse effect on resin members, and thus its use is not preferable. That is, use of ester-based oil is preferable from the viewpoint of high-temperature lubricability, but unpreferable from the viewpoint of adverse effect on the resin members. This is an actual problem.
In that case, it may be possible to use base oils of synthetic hydrocarbon oil series having no adverse effect on the resin members, but the viscosity index improving agent or its deteriorated product is less soluble in the synthetic hydrocarbon oil-based lubricating oil, so these insoluble matters will separate out, clog the pores of bearings and disturb the lubricating oil feeding to the sliding parts, often resulting in cutting of oil films and lubricating failure.